1. Field of the Invention
The present invention relates to an antireflection film for effectively preventing external reflection, and a display apparatus having the antireflection film.
2. Description of the Related Art
A glass plate is generally used as a substrate for a window glass, a show window, or the display surface of a display device. The glass plate sometimes causes specular reflection of ambient light such as daylight or light from lighting units. For this reason, a reflection often occurs, resulting in deterioration in transparency. Especially in a display apparatus, if specular reflection occurs on the display surface, an image to be displayed on the display surface overlaps an image of a light source, a scene, and the like to cause considerable deterioration in image quality.
In a conventional method of preventing such reflection, a single-layer or multilayer optical film, i.e., an antireflection film, is formed on the substrate surface to prevent external reflection by using interference of light.
One well-known antireflection film is referred to as a 1/4-wave film. This 1/4-wave film will be described below.
When external reflection is to be prevented by a single-layer antireflection film, the following non-reflection conditions must be satisfied, provided that the refractive index of air is represented by n.sub.0 ; the refractive index of the thin film, n.sub.1 ; the refractive index of the substrate, n.sub.2 ; the thickness of the thin film, d; and the wavelength of light which is to be prevented from reflecting, .lambda. . EQU n.sub.1 d=.mu./4 (1) EQU n.sub.1.sup.2 =n.sub.0 n.sub.2 ( 2)
Since the thickness of the thin film which satisfies these equations (1) and (2) corresponds to 1/4 the wave-length of the light which is to be prevented from reflecting, the film is called a 1/4-wave film.
When the equations (1) and (2) are satisfied, the reflection of the light having a wavelength of .lambda. can be reduced to zero. When a glass substrate is used, n.sub.2 is 1.52, and the refractive index n.sub.0 of the air is 1.00. Therefore, the refractive index n.sub.1 of the thin film must be set to be 1.23. The most practicable low-refractive-index material of currently known thin film materials is MgF.sub.2. The refractive index of MgF.sub.2 is 1.38, which is larger than the refractive index (n.sub.1 =1.23) defined by the non-reflection conditions. For this reason, it is impossible to completely prevent external reflection by using only a single-layered low-refractive-index thin film.
Under the circumstances, attempts have been made to prevent reflection by forming a two-layer antireflection film consisting of lower and upper layers formed on a substrate. With this film, the following non-reflection conditions must be satisfied, provided that the refractive index of air is represented by n.sub.0 ; the refractive index of the upper layer, n.sub.3 ; the refractive index of the lower layer, n.sub.4 ; the refractive index of the substrate, n.sub.2 ; the thickness of the upper layer, d.sub.1 ; the thickness of the lower layer, d.sub.2 ; and the wavelength of light which is to be prevented from reflecting, .lambda.. EQU n.sub.3 d.sub.1 =.lambda./4 (3) EQU n.sub.4 d.sub.2 =.lambda./4 (4) EQU n.sub.2 n.sub.3.sup.2 =n.sub.0 n.sub.4.sup.2 ( 5)
According to these equations (3), (4), and (5), if the substrate is a glass plate, since n.sub.2 =1.52 and n.sub.0 =1.00, external reflection can be prevented by selecting materials for the lower and upper layers such that a refractive index ratio n.sub.4 /n.sub.3 is set to be 1.23.
It is known that an antireflection film having not only two layers, but three or more layers may be used for controlling the reflection in a wide range. More specifically, the thickness of an antireflection film is determined by the wavelength of the light. Therefore, theoretically, by use of a multilayer having an N-number of layers, the reflectance relating to each of an N-number of wavelengths can be decreased.
Published Unexamined Japanese Patent Application No. 3-261047 discloses a conventional method, wherein a two-layer antireflection film is used, and the refractive index of the lower layer is controlled by adjusting the porosity thereof. Further, it is proposed to add pigments to the two-layer antireflection film so that it will have an optical filter effect.
As described, it is known that a multilayer antireflection film should be formed on a glass substrate surface in order to reduce external reflection.
Regardless of its type, the conventional multilayer antireflection film has a structure in which materials having high and low refractive indices are formed on a glass substrate. This is because the refractivity of the multilayer film had to be set at a certain value in order to satisfy the non-reflection conditions for a specific wavelength. However, there are only a limited number of practicable low- and high-refractivity materials, and appropriate combinations must be selected from the limited number of the materials. Examples of the high-refractivity material are TiO.sub.2, ZrO.sub.2, BaO, SnO.sub.2 ; examples of the low-refractivity type are MgF.sub.2, SiO.sub.2, and SnO.sub.2. The selection must be made from these materials.
In addition, when the layers of the multilayer film are made of different materials as described above, the problem of adherence between layers arises. As a result, the conditions for forming a film must be complicatedly restricted.